ARTICLES 

 THE DENSITY OF LIQUIDS 



By JOSEPH REILLY, M.A., D.Sc. (N.U.I.), and Prof. W. N. RAE, M.A. (Cantab.) 



One of the most frequent and important of the physical deter- 

 minations carried out in the laboratory is that of the density 

 of liquids and solutions. The chief methods in use are (a) the 

 pyknometer method ; (b) the method of hydrostatic weigh- 

 ing ; (c) the hydrometer method. By the pyknometer method 

 it is possible to obtain the density of a liquid correct to one 

 part or even less in a million, when great precautions are 

 taken in weighing, in regulating temperature and in allowing 

 for the effects of humidity. By Lamb and Lee's (i) refine- 

 ment of the hydrostatic method it is possible to obtain results 

 correct to one unit in the seventh decimal place. It is ques- 

 tionable, however, whether the amount of impurity in a liquid 

 can ever be reduced to such a small amount. The hydrometer 

 lends itself to a degree of accuracy which does not exceed one 

 part in a thousand. 



In making up solutions for density determinations, the 

 components must be always weighed out, and in the calcula- 

 tion of the components of the solution, weights corrected to 

 vacuum should be used. Methods, such as fractional distilla- 

 tion, freezing, etc., to be used in purifying the substances 

 will be dictated by their chemical nature and the nature of 

 the impurities known or suspected to be present. No general 

 rules can therefore be laid down, except that it is advisable 

 to consult the literature on previous determinations with the 

 same substance, before deciding on the method of purifica- 

 tion to be adopted. Suitable precautions must also be taken 

 to prevent contamination after purification, e.g. with moisture 

 in the case of hygroscopic substance. 



The vessel used in the pyknometer method is some form 

 of specific gravity bottle ; a large number of different forms 

 have been suggested, of which the most useful are perhaps : 



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